CONSTRUCTION OF A BACTERIOPHAGE-RESISTANT DERIVATIVE OF LACTOCOCCUS-LACTIS SUBSP LACTIS-425A BY USING THE CONJUGAL PLASMID-PNP40

Lactococcus lactis subsp. lactis 425A is an atypical strain which excretes a high concentration of alpha-acetolactate when grown in milk. The conjugative lactococcal plasmid pNP40, which encodes phage and nisin resistance, was introduced to strain 425A by conjugation, using resistance to phage and nisin as a selection. No phage-nisin resistance mutants were encountered. Transconjugants display complete resistance at both 21 and 39-degrees-C to those phage previously identified as lytic for 425A. Transconjugants lose their resistance characteristics when spontaneously cured of pNP40. The commercially important property of 425A-production of high levels of alpha-acetolactic acid-is unaffected by the presence of pNP40.